The present invention concerns a display assembly including at least two superposed display devices and more particularly an assembly of this type including means allowing one of the display devices to appear selectively to an observer to the exclusion of the other.
The present invention also concerns a timepiece including a display assembly of this type and more particularly such a display assembly wherein an analogue display device is combined with a digital display device.
French Patent No. 2 462 283 already discloses an electronic watch including a case in which both an analogue display device and a digital display device are arranged. The analogue display device includes an hour hand and a minute hand which move above a dial in a conventional manner, while the digital display device includes a transparent liquid crystal cell arranged in front of the analogue display device which it completely covers. In this Patent, this cell forms the watch crystal. The digital display device allows alphanumeric characters to be displayed, for example the day of the week and the date in a dark colour on a light background or, if required, in a light colour on a dark background, the dial and the hands of the analogue display device still being visible through the digital display device. When there is no data displayed by the digital display device, the watch thus has the appearance of a conventional watch, i.e. the hands and the dial are completely visible through the transparent digital display device forming the crystal. Conversely, when data is displayed by the digital display device, this data is displayed in superposition to that of the analogue display device. This consequently makes it difficult to read the data displayed by the cell and it becomes more difficult the greater the density of data displayed by the cell. Since the current trend is to make multi-functional watches of the diary, pager type etc., in which the digital display device has to display simultaneously a large number of data, this problem is all the more critical.
The object of the present invention is to overcome the drawbacks of the aforementioned prior art by providing a display assembly including at least two superposed, respectively lower and upper, display devices, in which it is possible to make data displayed by one of the display devices appear selectively to an observer to the exclusion of the data displayed by the other, with the data displayed by the display device in question being able to be easily read.
Another object of the present invention is to provide a display assembly wherein the upper display device is a liquid crystal display device having an improved display contrast.
A further object of the present invention is to provide a timepiece fitted with such a display assembly, having an improved aesthetic appearance.
The invention therefore concerns a display assembly including two superposed, respectively lower and upper, display devices, characterised in that the upper display device includes a double structure formed of a display cell arranged above an optical valve, said double structure being arranged so that said cell and said valve are transparent in a first state so as to make the lower display device visible, and that the cell displays an item of data and that the valve is opaque and reflective and at least partially masks the lower display device in a second state, control means supplying a control voltage to cause the display cell and the optical valve to pass from the first state to the second state and vice versa.
As a result of these features, it is possible to use either the lower display device or the upper display device selectively, without one altering the quality and legibility of the other""s display. In particular, if one chooses to use the lower display device, the upper display device can be switched into its first state in which it is totally transparent, so that the data displayed by the lower display device is perfectly visible. If, conversely, one chooses to use the upper display, the optical valve is then switched into the second state in which it becomes opaque and reflective to mask the lower display device totally, while the cell of the upper display device can display the desired data. This data can be displayed in a light colour on a dark background or in a dark colour on a light background as a function of the type of display cell used. In this way, any difficulty in reading the data resulting from the superposition of the lower and upper display devices is removed, which particularly improves the ability of the data displayed by the display device chosen to be read.
According to a first embodiment, the optical valve includes a twisted nematic type cell provided with a polariser placed in front of the cell, and a quarter-wave plate associated with a cholesteric film placed in succession behind the cell, the optical valve or the lower display device further including a second quarter-wave plate placed behind the cholesteric film and the lower display device including a metal reflector placed behind said second quarter-wave plate.
The passage of light through the second quarter-wave plate allows the circular polarised light, exiting the cholesteric film, to be converted into linear polarised light for which the metal reflector is more efficient. This structure allows the light reflected by the display assembly to be homogenised and the colour thereof to be less dependent on the wavelength of the light.
According to a second embodiment, the optical valve includes a twisted nematic type cell provided with a polariser placed in front of the cell, and a quarter-wave plate associated with a cholesteric film having a first helical direction, placed in succession behind the cell, the lower display device including a cholesteric mirror having an opposite helical direction to that of said cholesteric film.
As a result of this structure, the whole of the light passing through the cholesteric film, which represents half of the incident light, is reflected by the cholesteric mirror, which improves the brilliance of the display assembly. In the case of a wristwatch or suchlike, the cholesteric mirror can advantageously form the watch dial. Another advantage of this structure lies in the fact that one can choose cholesteric mirrors of different colours, which allows coloured display assemblies to be made.
According to a third embodiment, the optical valve includes a twisted nematic type cell provided with a polariser placed in front of the cell, and a quarter-wave plate associated with a cholesteric film placed in succession behind the cell, the lower display device further including a half-wave plate (xcex2) and a second cholesteric film identical to the first.
As a result of this structure, two identical optical elements associated with the display cell can be used, which constitutes an advantage from the practical point of view.
According to an advantageous embodiment, the display cell and the optical valve are in a transparent state in the absence of voltage applied by the control means. Thus the data displayed by the lower display device is permanently visible without the upper display device consuming power. This is particularly advantageous within the scope of an application to a portable object such as a wristwatch.